Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. Such implantable devices are typically powered by a battery. When the battery's useful life has been exhausted, the implanted device is typically explanted and replaced before the cessation of therapy. Therefore, it is often useful to know how much battery capacity has been used and/or how much battery capacity remains. Improved systems and methods for battery management and determining depth of discharge for batteries in implantable medical devices are needed.